A micromechanical analysis of time-dependent behavior based on subcritical damage in claystones

In this paper, a micromechanical model is proposed for time-dependent strain in claystones. The representative volume element of claystone is characterized by a three-phase composite. Quartz and calcite grains are embedded inside clay matrix. The time-dependent strain is related to subcritical propagation of microcracks inside the clay matrix. A two-step homogenization procedure is proposed. A micromechanics-based isotropic damage model is first formulated for the subcritical growth of microcracks inside the clay matrix. Different homogenization schemes are considered by taking into account opening and closure of microcracks. Two driving forces are used respectively for the subcritical evolution of damage under tensile and compressive stresses. Then the macroscopic properties of the claystone are determined by a second-level homogenization procedure using three different schemes. The numerical algorithm for numerical implementation of the proposed model in standard finite element code is proposed. Numerical evaluations of the proposed model are performed through simulations of creep tests under different loading paths and using different homogenization schemes. Finally, comparisons between numerical results and experimental data are presented.